In my prior U.S. Pat. No. 5,477,094 there is shown a magnetic coupler in which a magnet rotor unit is straddled by two conductor rotors which are connected together to rotate as a conductor rotor unit on one shaft while the magnet rotor unit is mounted to rotate on a second shaft. The magnet rotor unit has a set of permanent magnets arranged with their opposite poles spaced by air gaps from ferrous-backed electroconductive rings mounted on respective of the conductor rotors. Rotation of one of the two shafts results in rotation of the other shaft by magnetic action without there being any direct mechanical connection between the shafts.
Such magnetic couplers disclosed in my U.S. Pat. No. 5,477,094, have the advantage that if the load freezes up, for example, the input shaft from the prime mover remains free to rotate, and hence the prime mover is not damaged by the difficulty with the load. However, if this condition is permitted to continue indefinitely the resulting abnormally high slip between the conductor and magnet rotors may result in a heat problem likely to damage the rotors. One solution is to provide a speed sensing system operative to shut down the prime mover when the output shaft stops or when the relative speed between the input and output shaft exceeds a preset amount. However, such a speed sensing system does not provide ideal relief in situations wherein the magnetic coupler is overloaded because of a temporary excess load condition, and not by a freeze-up of the load.
My prior U.S. Pat. No. 5,477,094 also discloses the concept of having two magnet rotors rather than a single magnet rotor unit, with each magnet rotor having a respective set of permanent magnets spaced by an air gap from one of the electroconductive elements presented by the conductor rotors. The two magnet rotors float on the related shaft in that they are axially moveable relative to one another and are spring biased apart. It is now recognized by me that if such a coupler were to be overloaded, the two magnet rotors might move toward one another responsive to the resulting build-up in the slip between the magnet rotors and the conductor rotors to an extent where the current induced in the electroconductive elements results in a repulsion between the magnet rotors and conductor rotors which exceeds the attraction between the magnets on the magnet rotor and the ferrous backing of the electroconductive elements on the conductor rotors. The resulting increase in one or both of the air gaps could temporarily relieve the coupler from the overload condition. The two magnet rotors would then tend to move back toward the conductor rotors if the effect of the increased air gaps in reducing the repulsion between the magnet and conductor rotors is sufficient to permit the magnet attraction to the ferrous backing to dominate. This action could reset the coupler provided that in the meantime the load was sufficiently reduced.